Cataract is the leading cause of blindness globally and the second most common cause of visual impairment in the world. Age-related cataract is a multifactorial disease that involves complex interactions between environmental and genetic factors and, as in other complex diseases, these interactions are not fully understood and the genetic contribution varies. Twin studies, investigating the relative contribution of genetic versus environmental effects in age-related cataract, estimated the genetic contribution as up to 58%, whereas environmental effects accounted for 26% in cortical cataract. In nuclear cataract the contributions were 48% and 14% respectively.Citation1,Citation2 Several studies show that the prevalence of cataract is higher in women compared to men at the same age. It has been suggested that the potential protective effects of estrogen are withdrawn when a dramatic alteration in estrogen levels is introduced at menopause, resulting in increased risk of cataract in women.Citation3,Citation4 This gender-dependent risk of developing lens opacities has led to the interest of the role of estrogens in cataractogenesis.
The eight single nucleotide polymorphisms (SNPs) chosen in this study were either functional or disease-associated polymorphisms found in genes encoding estrogen receptors (ESR1, ESR2) or enzymes involved in the biosynthesis (CYP17A1, CYP19A1), bioavailability (CYP1A1) and inactivation (COMT) of estrogens. This candidate SNP-based case-control association study included 491 patients with age-related cataract and 185 controls. The study subjects were recruited from two ophthalmic clinics in Tartu and the South Estonian area and cataracts were determined and classified into the following subtypes; cortical cataract (n = 151), posterior subcapsular cataract (n = 119), nuclear cataract (n = 75) and mixed cataract (n = 146). The classification was done prior to surgery, using biomicroscopy and ophthalmoscopy, and patients with secondary cataracts were excluded. Subjects without cataract, uveitis, and glaucoma were included as controls. Data on smoking (current and former smoking habits) was obtained for all individuals and age was reported at the time of surgery or when included in the study as controls. All the SNPs were genotyped using genomic DNA extracted from whole blood samples. TaqMan SNP Genotyping Assays and TaqMan Drug Metabolism Genotyping Assays (Applied Biosystems, Foster City, CA, USA) were used according to the TaqMan Allelic Discrimination technology.
Descriptive statistics showed no differences between the cataract and control group except for a significantly lower mean age in the control group. Although there was no significant difference in smoking habits in the overall cataract group compared to controls, the nuclear cataract subtype had a higher frequency of current smokers; 24% of patients with nuclear cataract were current smokers as opposed to 9.7% of controls. All the SNPs had a Hardy-Weinberg equilibrium p-value of > 0.1. Single marker association tests were performed using logistic regression in an additive model (homozygote for major allele = 0, heterozygote = 1 and homozygote for minor allele = 2), including relevant risk factors for age-related cataract; age, sex and smoking, as covariates in the analyses. None of the SNPs showed significant associations with risk of cataract when comparing all cases and controls, neither in the univariate analyses nor when adding the known risk factors (age, sex and smoking) as covariates in the analyses (). After stratification by cataract subtype, the nuclear cataract (NC) group had a significantly lower minor (C) allele frequency of rs10046 compared to the control group (p = 0.026). After including covariates in the analysis the adjusted odds ratio (OR) was 0.56 with a 95% confidence interval (CI) of 0.35 to 0.90, p = 0.016. However, after correcting with Bonferroni for multiple testing of the eight SNPs in the four different cataract subtypes, this finding was no longer significant (p = 0.51).
TABLE 1. Genotype frequencies of estrogen-related SNPs in Estonian patients with age-related cataract and controls.
The biological effects of estrogens are many and they can exert their actions by binding to their receptors ERα and ERβ encoded by two separate genes, ESR1 and ESR2. We studied two well-characterized polymorphisms in ESR1, rs2234693 and rs9340799, also referred to as PvuII and XbaI restriction sites, which have been associated with decreased estradiol (E2) serum levels in postmenopausal women and several diseases such as age-related macular degeneration (AMD).Citation5 The Rotterdam study showed an increased risk of open-angle glaucoma in men carrying both the ESR2 polymorphisms investigated here, rs4986938 and rs1256031.Citation6 The latter SNP has also been associated with higher intraocular pressure in female patients with primary open-angle glaucoma.Citation7 However, none of the four polymorphisms studied in ESR1 and ESR2 were associated with increased risk for cataract in our study subjects.
Many of the polymorphisms chosen in this study have been shown to influence estrogen metabolism and circulating estrogen levels and are found in genes encoding cytochrome P450 (CYP) enzymes such as CYP1A1, CYP17A1 and CYP19A1, which are some of the major isoforms of these enzymes being crucial in estrogen metabolism. CYP17A1 is involved in the biogenesis of estrogens by converting progesterone and pregnenolone to androstenedione and dehydroepiandrosterone (DHEA). CYP19A1, also called aromatase, is then involved in the final step of the biosynthesis of estrogens when androstenedione and testosterone are converted, through aromatization, into estrone (E1) and estradiol (E2), respectively. Two of the polymorphisms included in this study, rs743572 (CYP17A1) and rs10046 (CYP19A1), have been shown to influence endogenous E1 and E2 levels.Citation8,Citation9 CYP1A1 is involved in the hydroxylation of E1 and E2 into their respective catechol estrogens, which results in lowered estrogenicity. The SNP, rs1048943, in CYP1A1 can increase catalytic activity thus forming more hydroxylated estrogen metabolites from E1 and E2.Citation10 Catechol-O-methyltransferase (COMT) is involved in estrogen inactivation by conversion of catechol estrogens through O-methylation into methoxy metabolites with low or no affinity for ERs. In this gene we genotyped rs4680, a well-studied functional polymorphism, that results in reduced enzyme activity due to a valine-to-methionine substitution. It has been reported that postmenopausal women after intake of exogenous E2, had higher serum E2 levels compared to women with the wild-type (Val/Val) genotype.Citation11
There are inconsistent results whether or not polymorphisms in estrogen-metabolizing genes affect circulating estrogen levels and disease susceptibility in females.Citation12 However, none of the SNPs studied in CYP1A1, CYP17A1 and COMT were associated with increased risk for cataract in our Estonian subjects when considering all types of cataract. Conflicting data exist for rs10046 in CYP19A1; different effects of the C and T alleles on serum E1 and E2 levels have been reported as well as conflicting data on its influence on risk of breast cancer.Citation8 Interestingly, we found an association with the minor (C) allele in rs10046 and risk of nuclear cataract in our studied population but after correcting for multiple testing this association was no longer significant. In summary, in our studied population there was no evidence of polymorphisms in genes encoding estrogen receptors or enzymes involved in the biosynthesis, bioavailability and inactivation of estrogens, being associated with risk of age-related cataract.
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
This work was supported by grants from the Swedish Research Council (#2011-3132), Swedish government (“Agreement concerning research and education of doctors”; ALF-GBG-145921), Göteborg Medical Society, the Knut and Alice Wallenberg Foundation, Marianne and Marcus Wallenberg Foundation, Dr Reinhard Marcuses Foundation, Konung Gustaf V:s och Drottning Victorias Frimurarestiftelse, Hjalmar Svensson Foundation, Greta Andersson Foundation, Herman Svensson Foundation, Ögonfonden, De Blindas Vänner and Kronprinsessan Margaretas Arbetsnämnd för Synskadade.
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